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Heap Leaching

Demonstration of the Application of Unsaturated Zone Hydrology for Heap Leach Optimization
Industrial Research Assistance Program Contract #332407

Heap leaching has become a widely used method of mining low-grade gold, silver, copper, and uranium ores.

One of the key components to success in heap leaching is favorable heap hydrology. Heap leach piles are unsaturated systems and must be addressed as such, by applying the knowledge and tools of unsaturated soil science to design and optimize operation.

This report presents the results of a laboratory column study completed to further characterize the nature of preferential flow within heap leach piles and its impact on mineral leaching. Numerical modelling of the column tests was completed to demonstrate the potential of developing a successful model for the prediction of heap leach performance using an unsaturated hydrology model as a basis. The objective was to demonstrate the application of unsaturated zone hydrology to aspects of heap and dump leach hydraulic dynamics.

Layers of coarse and fine textured ore inevitably develop within heap and dump leach piles as natural processes segregate coarse and fine material during material placement. Segregation of heap leach material will occur regardless of whether the material is agglomerated or non-agglomerated. Clearly, segregation is significantly reduced for agglomerated heap leach materials. However, due to the methodologies employed to place the material, segregation of agglomerated heap leach material will occur. Run-of-mine (ROM) material (i.e. dump leaching) will definitely segregate. Under such conditions leaching solution flows preferentially in the more conductive layer, potentially leaving areas within the heap unleached. The preferred flow path is not dependent entirely on the physical properties of each layer, but also on the stress state and resulting degree of saturation, and therefore the solution application rate. For this reason either the coarse or the finer material can be the preferred flow path. Research completed as part of this study investigated flow rates required to create both preferential flow conditions, and the impact that extreme cases of preferential flow have on the leaching of adjacent layers. This complicated flow and transport problem was then modelled to demonstrate the effectiveness of existing unsaturated zone hydrology numerical models. The objective was to illustrate tools exist for understanding key processes and characteristics that control performance and can improve design and operation.

Column testing revealed that solution application rates greater than the saturated hydraulic conductivity of the finer material resulted in preferential flow in the coarser layer. The preferred flow path became the finer textured material when application rates were less than the saturated hydraulic conductivity of the fine material.

The coarse and fine textured layers of material in the columns were also “spiked” with salt prior to placement in the column and subsequent leaching. Under preferential flow conditions it was found that 30% of the salt within the less conductive material could be leached from that material by the horizontal movement of water into the more conductive material. This result was dependent on the amount of infiltration into the less permeable layer at the top of the column. An ineffectual volume flowed preferentially if little water originally entered the less conductive material.

Noteworthy is that for the segregated components investigated, the preferred flow path was the finer textured material for application rates common to heap leaching. The implication is that a typical operational response to poor recovery is to increase the solution application rate. However, the effect of this response will potentially decrease recovery because it enhances the potential of the solution to flow in the coarse textured material. It is also a common occurrence for flow to be transported laterally and exit from the sides of the pile under the higher solution application rates, as a result of internal segregation, compaction, consolidation, and generally differences with in situ density. The worst-case scenario is a massive slope failure and significant reduction in recovery. The answer for balancing the solution application rate and recovery is rooted in the hydraulic dynamics of the heap or dump leach, which by definition is an unsaturated zone hydrology problem.

The numerical modelling effort successfully reproduced both preferential flow patterns. The preferential leaching which resulted was also successfully modelled. In this way existing unsaturated soil science modelling tools proved to be valuable in predicting the performance of heap leach piles. The study clearly demonstrated that unsaturated zone hydrology should be used as a basis for heap and dump leach performance.

This project was initiated by O’Kane Consultants Inc. and was completed in partnership with the Unsaturated Soils Group, University of Saskatchewan, and M.D. Haug and Associates Ltd. The project was funded in part by O’Kane Consultants Inc., as well as through a grant from the Industrial Research Assistance Program under Contract # 332407.